THE MINERALOGY OF MONOTONOUS BASALTIC ANDESITES OF THE CURRENT ERUPTION OF ARENAL VOLCANO, COSTA RICA: CONSTRAINTS FOR TIMING AND INTERACTION OF COMPOSITIONALLY CONTRASTING EVOLVED AND MAFIC MAGMAS

Arenal volcano is an extraordinary example of an arc volcano erupting compositionally monotonous basaltic andesites continuously over decades since 1968. Mineral assemblages consist of plagioclase (plag), clinopyroxene (cpx), orthopyroxene (opx), and spinel. Olivine occurs sparsely throughout the current eruption but hornblende is restricted to the first 2 years.

Phenocrystic cpx displays large compositional ranges from Mg# higher than 85 to as low as 70. Mg# correlates positively with Cr2O3 and Al/Ti ratios. Most opx phenocrysts are compositionally restricted ranging in Mg# from 72 to 74, occasionally reaching 76. The majority of opx inclusions in cpx are similarly evolved however sparse inclusions have Mg#’s as high as 87. Occasional olivine phenocrysts are typically rimmed by opx and are mostly evolved ranging from Fo72 to Fo78. Yet, a Fo90 olivine inclusion was found next to high Mg# opx (~86) within a cpx grain of high Cr2O3 (0.3 wt.%) and high Mg# (82). Plagioclase is texturally complicated with a large compositional range of ~An94 to An55. Plag inclusions in cpx indicate high An plag (An >80) occurs in cpx with lower (± 77) Mg# and high (±83) Mg#; but low An plag (< 73) inclusions have only been found in low Mg# cpx (< 74). Spinel is compositionally also wide ranging from Cr-Al-rich spinel inclusions occurring in Cr-rich and high Mg# cpx compared to titanomagnetite found as inclusions in Cr-poor and low Mg# cpx or in the groundmass.

Compositional ranges combined with frequency distributions indicate that the current eruption has been mainly tapping a magma reservoir that crystallized low Mg# mafic phases (< 78), titanomagnetite, but possibly a wide range of plag compositions. High Mg# mafic phases and Cr-rich spinel indicate some portions of crystals are derived from more mafic (basaltic?) magmas. Crystallization during earlier centuries followed by incorporation of crystals into currently tapped magma to explain the more mafic crystal components is to some extent likely, however high Mg# cpx near euhedral phenocryst rims also requires that mafic magma has been feeding into the subvolcanic reservoir during the current eruption. Thus at Arenal, individual phenocrysts and their inclusions preserve crystallization events from "unseen" mafic magmas that may precede eruption by more than centuries to less than months.